1. Field of the Invention
The present invention relates generally to an optical system, and more particularly to compact image-forming and ocular optical systems having a wide angle of field as well as an improved resolving power.
2. Discussion of Related Art
In recent years, a helmet or goggle-type of head- or face-mounted image display devices have been developed for virtual reality purposes or with a view to allowing individuals to enjoy wide-screen views.
For instance, JP-A-2-297516 discloses an optical system made up of a 2D display device 21 for displaying images, an objective collimating lens 22 and a parallel transparent plate 23 having off-axis paraboloidal mirrors on both its ends, as shown in FIG. 21. Light rays leaving the 2D display device 21 for displaying images are converted by the objective collimating lens 22 into parallel rays, which are then subjected to first transmission through a parallel surface of the parallel transparent plate 23, reflection at the first paraboloidal mirror, some total reflections within the parallel transparent plate 23, reflection at the second paraboloidal mirror, and second transmission through the parallel surface of the parallel transparent plate 23 (8 reflections and 2 transmissions), whereby an intermediate image is formed on a point F for projection into an observer's eyeball 24.
U.S. Pat. No. 4,026,641 discloses an optical system wherein, as shown in FIG. 22, an object image displayed on an image display element 25 is converted by a transmission optical element 26 into a curved object image, which is in turn projected from a toric surface 27 into an observer's eyeball.
A problem associated with an image display device of the type wherein an image is relayed therefrom as shown in FIG. 21 is, however, that it needs not only an optical system but also a relay optical system, resulting in increases in the size and weight of the whole optical system as well as an increase in the amount of extension of the whole optical system from a head or face; it is not fit for a head- or face-mounted image display device.
The optical system for forming parallel rays as an intermediate image as well as the optical system for projecting an intermediate image into an eyeball produce some considerable aberrations because it is only the paraboloidal mirrors that has power.
The ocular optical system composed only of a concave mirror as shown in FIG. 22, too, produces some considerable aberrations and so is detrimental to image quality, even though the concave mirror is defined by a toric surface as shown in FIG. 22. To make correction for field curvature occurring at the ocular optical system, therefore, it is required to use the transmission optical element 26 such as a fiber plate. Even by use of the transmission optical system 26 and toric surface 27, however, it is impossible to make adequate correction for coma, and other aberrations.